FR: ajouté aux cahots

The following sentence is cited from l'Etranger:
Cette hâte, cette course, c'est à cause de tout cela sans doute, ajouté aux cahots, à l'odeur d'essence, à la réverbération de la route et du ciel, que je me suis assoupi.
I know that the author uses c'est...que... to emphasize the cause of the hero's sleepiness, but I fail to understand the underlined phrase leading by the past participle. I guess something is added to something (literal translation), making it worse, but what to what?